Method of bleaching mechanical wood fibers



March 29, 1949. R. MCEWEN 2,465,738

METHOD OF BLEACHING MECHANICAL WOOD FIBERS Filed Dec. 22, 1945 Y GRAPH I l k l v 2 l0 x l a '1 k "a g 6 Q a Q GRAPH H a o /0 a0 30 40 5b 60 CO/VJ/JTENCY INVENTOR Robe); L. NCEWen.

ATTORNEYS Patented 1949 UNITED STATES- PATENT OFFICE mrrnon or BLEAOHING MncmimcA L woon mans Robert L. McEwen, Willlamsvifle, N. Y., auignor to Buffalo Electro-Chemical Company, Inc.,

Tonawanda, N. Y.

Application December 22 1945, Serial No. 838,619

This invention relates to an improvement in the bleaching of mechanical wood fibers and particularly to the bleaching of such fibers with peroxide bleaching solutions under conditions bleaching solutions where the wood fibers are suspended in water to the extent of about 4% to 7% of the total mass; at such proportions the material is in the form of a slurry, is flowable and pumpable and water is present in a continuous phase.

The present invention deals with the brightening or bleaching of fibers of ground or mechanical wood wherein the fibers are not suspended in the bleaching solution but rather may be considered to be moistened with the peroxide bleaching solution. Under such conditions, it has been found in accordance with the present invention that very considerable increases in brightening action may be obtained by the employment of a given amount of active bleaching agent. Whereas Very small increments of brightness are obtained by treating suspensions of groundwood, at consistencies above about 18% and where water is no longer the continuous phase, the increment of brightness is considerably increased at particular temperatures when the pulp is treated not as a suspension but rather as fibers merely moistened with the brightening solution. The optimum consistency for obtaining maximum brightness is about 18% to-about 38%.

The following examples are given merely as illustrative of the invention but not as limitative thereof, since the invention may be applied with equal facility on substantially all kinds of mechanically prepared fibers whether they be spruce, balsam, pine or other types of wood.

EXAMPLE I Spruce groundw-ood pulp, prepared by grinding spruce logs on the conventional pocket grinding equipment, was subjected at specified densities at 40 C. to the bleaching action of an aqueous alkaline peroxide solution. The following table shows the brightness increment obtained, that the increase in units of brightness measured in percent reflectance of an air-dry sheet of null 3 Claims. (Cl. 8-10!) prepared from the treated pulp at consistencies varying from 6% to 15% on pulp suspensions and irom 18% 'to 50% on moistened fiber. The following table gives the results obtained:

Table 1 Maximum Consistency 33%.? Increment EXAMPLE II Table II Maximum Consistency pggfitfi ss Increment The treating solution consisted of 2% hydrogen peroxide volume (2'7.5% by weight) and 8% sodium silicate, 42 B., all chemicals being expressed as amount on oven dry weight of fiber. Bleaching solutions of hydrogen :peroxide, as is well known, may be made from hydrogen peroxide itself by the addition of alkalizing material, such as sodium silicate, caustic soda, alkaline phosphates, sodium carbonate and others, or may be made from the alkali metal peroxides or even alkaline earth metals, such as perborates, percarbonates and the like, all classed herein as peroxides due to the fact that hydrogen peroxide is formed in aqueous solutions. It may be necessary in some instances, as in the case of alkaline earth metal peroxides or alkali metal peroxides, to neutralize a portion or all of the alkali with acids. Although specific amounts of :peroxide are indicated as used in the examples above, greateror lesser amounts may be found to be suitable, depending upon the kind of fiber treated.

The results of the experiments of Example I as set forth in Table I are shown in graph I of the drawing, while the results of Example II as set forth in Table II are shown in graph II of the drawing.

Referring more particularly to graph I, it will be noted that where pulp suspensions are treated with peroxide solutions as brightening agents, very slight increases in brightness are obtained as the amount of pulp suspended in the water is increased. Howeven'from about 18% to 20% of pulp, the rate of increase of brightness or the slope of the curve as shown in graph I increases very sharply, reaching a maximum just short of about 40% and then begins to fall off.

Graph II contains two portions similar to those in graph I; namely, a slow rise to maximum at about 12% to 15% consistency, then a rapid rise of the increase in brightness when the pulp consistency is greater than about 18% rising to a maximum somewhat short of 40% and then falling 011'.

'From the above, it will be seen that a very marked increase in brightness can be obtained by operating upon moistened fibers in contrast to the fiber suspensions described in the prior art.

In carrying out the operation of the present invention, the mechanical pulp produced in com ventional fashion and a low consistency on the order of 3% is suitably dewatered to produce a moist fibers. These fibers are treated in any conventional fashion with the peroxide bleaching" solution and permitted to bleach. Depending upon the type of wood from which the fiber was prepared, and the temperature at which the peroxide moistened fibers are maintained, optimum brightening is obtained in from about 1 to 4 hours. Generally, the brightening action is carried out at "temperatures below 80 C. and

preferably in the range 40 C. to C4 main tenance of temperatures above about C. does not produce properly brightened fibers.

What is claimed is:

1. The method of bleaching fibers of ground 4 wood which comprises moistening the fibers with an alkaline peroxide bleaching solution at fiber consistencies of 20 to bleaching the fibers at said consistencies and maintaining the temperature of the so treated fibers below about 80 C. and above about 40 C. during the bleacing treatment.

2. The method of bleaching fibers of ground wood which comprises moistening the fibers with an alkaline peroxide bleaching solution at fiber consistencies of 20 to 35%, bleaching the fibers at said consistencies and maintaining the temperature of the so treated fibers between about 40 C. and 50 C. during the bleaching treatment.

3. The method of bleaching ground wood fibers which comprises incorporating suflicient amount of an alkaline peroxide bleaching solution in the fibers to obtain 20 to 35% consistency, bleaching the fibers at said consistency and maintaining the temperature in the neighborhood of 40 C. during the bleaching operation.

ROBERT L. McEWEN.

REFERENCES CITED The following references are of record in the tile of this patent:

UNITED STATES PATENTS Paper Industry," No. 19, Nov. 3, and 180.

Paper Trade Journal, vol. 121, i945, Tappi section, pages 133 

